This invention relates generally to sensors for detecting electrically conductive particulate matter, such as soot, and more particularly to a method and system for diagnosing potential failure modes in such sensors.
Incomplete combustion of certain heavy hydrocarbon compounds, such as heavy oils, diesel fuel, and the like may lead to particulate formation (e.g., soot). In the operation of internal combustion engines, excessive particulate formation can lead to “smoking” of the engine, which causes air pollution even though the carbon monoxide, hydrocarbons, and other pollutant components of the gaseous state exhaust emissions may be relatively low. Emission regulations require many engines to limit the levels of particulate emissions, and various control technologies such as diesel particulate filters (DPF) have been employed for this purpose.
In order to monitor the emission of particulate matter in the exhaust streams of certain types of internal combustion engines, e.g., to assess the effectiveness of DPF's, it is known to provide a particulate sensor system for detecting the level of particulate concentration emitted from an exhaust gas. Various particulate sensors have been proposed, including those shown in U.S. Pat. No. 4,656,832 issued to Yukihisa et al., U.S. Pat. No. 6,634,210 issued to Bosch et al., U.S. Pat. Publ. No. 2008/0283398 A1, U.S. Pat. Publ. No. 2008/0282769 A1, and U.S. Pat. Publ. No. 2009/0139081 A1, the disclosures of each of which are hereby incorporated by reference in their entirety.
Particulate sensors such as those described above generally have a pair of spaced apart sensing electrodes disposed on a substrate. The sensing electrodes are coupled to a measurement circuit by way of electrically conductive leads. The operating principle of the particulate sensor is based on the conductivity of the particulates (e.g., soot) deposited on (or over) the sensing electrodes. The electrical resistance between the sensing electrodes is relatively high when the sensor is clean but such resistance decreases as soot particulates accumulate. These sensors also have a heater that can be selectively activated to burn off the soot particulates to “reset” the sensor to a known, base “clean” state.
However, for diagnostic purposes, it can be difficult to distinguish between various states that may occur during various engine operating conditions, such as between: (i) a faulty state such as when there is an electrical open circuit in the wiring leads, which presents as a very high resistance between the sensing electrodes, and (ii) a normal state, such as when a sensor has just been cleaned, which also presents as a very high resistance, or between (i) a false positive state such as where the sensor presents a low resistance due to some cause other than soot particulates, e.g., water vapor condensate on the electrodes, and (ii) an actual positive state such as where soot particulates on the electrodes lead to a low resistance measurement.
Accordingly, there is a need for particulate sensor diagnostics that can accurately distinguish between sensor states during various engine operating conditions.